1. Field of the Invention
The present invention relates to an optical pickup for an optical data storage medium having narrow tracks. More particularly, the invention relates to an optical pickup which can read information stored in an optical data storage medium, even when a signal rate in the optical data storage medium, i.e., the distance between a target pit track and an adjacent pit track, is shortened.
2. Description of the Related Art
The size of a beam spot of a light beam incident on an optical data storage medium is directly related to the size of a mark or pit recorded on the optical data storage medium. Therefore, it is necessary to produce a minimum possible size of the beam spot, in order to increase a data storage capacity of the optical data storage medium. To produce a small beam spot, an optical data storage system using a short-wavelength light source and an objective lens having a large numerical aperture (NA) is under development. Together with the development of the optical data storage system, a method of increasing a data storage capacity in an optical data storage medium by diminishing a size of the pit which is a minimum physical recording unit, is also under development. As a result, optical data storage media have been developed from a compact disc (CD) using an infrared light source of 780 nm wavelength and an objective lens of 0.45 NA to a digital versatile disc (DVD) using an infrared light source of 650nm wavelength and an objective lens of 0.6 NA. Also, a track pitch is greatly reduced together with a change in the specification from CD to DVD.
To increase the storage capacity in the near future, an optical data storage medium having a shorter track pitch should be used. In this case, a reproduced signal severely deteriorates due to a signal interference between adjacent tracks. In an optical data storage medium, a track crosstalk is used for measuring a degree that a signal recorded on a target track deteriorates due to a signal stored in an adjacent track. In case of a disc of a ROM (read only memory) type, a required track crosstalk is a value of -30 dB or less. A conventional optical pickup which meets this requirement will be described with reference to FIG. 1.
The optical pickup shown in FIG. 1 uses three light beams in order to reproduce information stored in an optical disc 6. The optical pickup includes a light source 1 which is, e.g., made of a laser diode, a diffraction grating 2, a beam splitter 3, a reflective mirror 4, an objective lens 5, a light reception lens 7 and an optical detector 8. The diffraction grating 2 diffracts a light beam emitted from the light source 1 to divide the emitted light beam into three light beams. The three light beams are reflected by the beam splitter 3 toward the reflective mirror 4. The reflective mirror 4 reflects the three incoming light beams toward the objective lens 5. The objective lens 5 focuses the three incoming light beams on an information recording surface of the optical disc 6. As a result, the three light beams are focused on neighboring three tracks as three beam spots (a left-side beam spot, a main beam spot and a right-side beam spot). FIG. 2 shows the main beam spot and left-side and right-side beam spots formed in a target track and two (left-side and right-side) tracks adjacent to the target track, respectively, as circular shapes. In FIG. 2, the target track, the left-side track and the right-side track are determined based on the proceeding direction of the optical pickup of FIG. 1.
The light beams reflected from the neighboring three tracks pass through the objective lens 5, are reflected by the reflective mirror 4, and then proceed toward the beam splitter 3. The beam splitter 3 enables the three light beams reflected from the reflective mirror 4 to proceed toward the light reception lens 7. The light reception lens 7 focuses the three light beams incident from the beam splitter 3 on the optical detector 8. The optical detector 8 includes three photodiodes individually corresponding to the three light beams. The beam spots formed on the optical surfaces of the three photodiodes are shown in FIG. 3. In FIG. 3, the main beam spot formed in the target track is detected in a first photodiode, the right-side spot formed in the right-side track with respect to the target track is detected in a second photodiode, and the left-side spot formed in the left-side track with respect to the target track is detected in a third photodiode.
An electronics circuit (not shown) connected to the optical detector 8 uses electrical signals corresponding to the beam spots formed in the three photodiodes to detect signals of the adjacent tracks imposed on a signal read from the target track. Thus, the electronics circuit removes the signals imposed by the adjacent tracks from the read signal of the target track, so that the information stored in the target track can accurately be obtained.
However, the conventional system divides a light beam incident to an objective lens into three light beams, in order to obtain signals from two tracks adjacent to a target track. Therefore, an intensity of the light beam which is used for acquisition of information from a target track is reduced. Consequently, a light use efficiency is lowered. Also, an optical axis of the diffraction grating 2 must be accurately adjusted in order to accurately focus the three light beams divided by the diffraction grating 2 on the neighboring three tracks. Further, a separate electronics circuit as described above is required to remove the signals imposed from the adjacent two tracks from the read signal from the target track.